This invention relates to particles having ultrathin coatings on their surfaces and to methods for making and using such coated particles.
Ceramics and metals are used in particulate form in a variety of industrial settings, such as in the electronics and structural advanced materials industries. It is often desirable to alter the surface properties of these particles while maintaining their bulk properties.
For example, in some cases the particles have reactive surfaces that can be attacked by the surrounding environment or which otherwise engage in undesirable reactions. In these cases, it is often desirable to passivate the reactive surfaces to inhibit these reactions from occurring.
Conversely, it is desirable in other situations to activate the particle surfaces for various reasons, such as to improve coupling to other materials (or between particles) or to promote desired chemical reactions. In this manner, it would be desired to provide for improved coupling at ceramic/polymeric, ceramic/metallic (cermet), or ceramic/ceramic (monolithic or composite) particulate interfaces. For example, boron nitride (BN) and aluminum nitride (AlN) particles have been developed as fillers for electronics packaging applications. These materials significantly enhance the thermal conductivity of polymer-based composite packages, while maintaining good electrical insulation. These properties are becoming increasingly important as faster and denser integrated circuits are being developed by the microelectronics industry. The high thermal conductivity of BN and AlN make them attractive candidates for filler materials. However, the surfaces of BN and AlN particles are relatively nonreactive and do not adhere well to the coupling agents commonly used with these epoxy polymers. This incompatibility with the polymer makes it difficult to load these materials at levels sufficient for use with newer, high-density integrated circuits. Thus, it is desirable to find a way to improve the adhesion of these particles to the polymer matrix and to incorporate more of these nitride particles into the packaging material without significantly decreasing the thermal conductivity of the particles.
Another example of the desire to modify surface properties of materials comes from the ceramics industry. The development of sintering methods has enabled the widespread use of advanced ceramic materials for various applications. Densification of a ceramic material through sintering can be achieved by several methods that involve heating constituent particles either with pressure (such as hot-pressing, hot isostatic pressing, or gas pressure assisted sintering) or without pressure (such as pressureless sintering). Pressureless sintering is a preferred method due to its low cost. However, it requires the development of specialized processing formulations that usually involve liquid phase sintering. It is important to be able to control the surface properties of the constituent particles during pressureless sintering densification. In addition, it is desirable in these sintering applications to obtain a uniform dispersion of sintering aids, and to disperse the sintering aid as finely as possible.
While currently practiced commercial methods such as wet chemistry, physical vapor deposition (PVD), chemical vapor deposition (CVD), and plasma-enhanced CVD (PE-CVD) offer outstanding coating processing for flat substrates and large particles where relatively thick and non-uniform coatings are acceptable, they do not allow the controlled nanocoating of individual ultra-fine particles. In certain cases, the currently practiced processes are line-of-sight dependent (e.g. PE-CVD), do not provide for a chemically bonded film to the substrate surface (e.g. PVD), leave residue on the surface (e.g. wet methods), and/or cannot control the thickness of a non-granular film at the angstrom level (e.g. CVD).
Thus, it would be desirable to provide a method by which the surface properties of particulate materials can be modified without significantly changing the bulk properties of the particulate material.